1. Field of the Invention
The present invention relates to a sheet creaser, a sheet conveyer including a conveying path on which the sheet creaser is provided, a sheet finisher including the sheet creaser, an image forming apparatus including the sheet finisher or the sheet finisher.
2. Description of the Related Art
In the field of image forming apparatuses such as inkjet printers, electrophotographic copiers, facsimile machines, and multifunction products (MFPs), sheet finishers that receive a set of sheet-like recording mediums (hereinafter, “sheets”) from an image forming apparatus and perform post-processing such as stapling have been widely used. With the development of multi-functional-sheet finishers, sheet finishers with both a side-stitch function and a saddle-stitch function have appeared. In most of the sheet finishers with the saddle-stitch function, a folding unit that folds the set of sheets includes at least one pair of rollers called pressure rollers and a plate member called folding plate. More particularly, the folding plate is aligned with a line to be folded of the set of sheets, and inserts the set of sheets into a nip between the pressure rollers. Thus, a crease is made along the line to be folded on the set of sheets with the nip.
Some folding units include a first pair of pressure rollers and a second pair of pressure rollers. The set of sheets is pressed twice with the first pressure rollers and the second pressure rollers, which makes a stronger crease.
However, even when the set of sheets is pressed twice, it is difficult to make a crease strong enough due to a short pressing time and a low pressing force. Because a rotation axis of the pressure rollers runs parallel to a direction perpendicular to a sheet conveying direction, a folded side of the set of sheets is pressed in the nip between the pressure rollers only for a short time. Moreover, because the pressure rollers nip the entire folded side at the same time, the pressing force on the set of sheets is distributed, i.e., the pressing force per unit area is low.
There has been disclosed a technology for making a stronger crease, in which a slide-pressing unit re-presses the folded side while sliding in a direction perpendicular to the sheet conveying direction.
Japanese Patent Application Laid-open No. 2003-341930 discloses a sheet finishing method of accumulating a plurality of sheets received from the image forming apparatus and saddle-stitching/half-folding the sheets. More particularly, after the sheets are saddle-stitched, the stitched sheets are inserted in between a pair of first pressure rollers in such a manner that a center line with respect to the sheet conveying direction is pressed by the folding plate. Thus, a crease is made on the sheets. After that, the crease is re-pressed by a second pressure roller that is sliding in the direction perpendicular to the sheet conveying direction in such a manner that a rotational axis of the second pressure roller is oblique with respect to the crease. Thus, the strong crease is made on the sheets.
In Japanese Patent Application Laid-open No. 2003-341930, a guiding member that is swinging upward guides the second pressure roller so that the second pressure roller moves up slantwise and then moves down onto the crease. The guiding member is swung by a driving force of a motor.
In a typical sheet creaser that makes the strong crease by re-pressing the folded side of the sheets with a slidable pressure roller, such as the second pressure roller disclosed in Japanese Patent Application Laid-open No. 2003-341930, sliding in the direction perpendicular to the sheet conveying direction, if the folded side of the sheets is thick, a load on the motor steeply increases when the slidable pressure roller slides up on the crease. This may results in a step-out of the motor.
In Japanese Patent Application Laid-open No. 2003-341930, the increase in load on the motor when the second pressure roller slides up on the crease is suppressed by the presence of the guiding member. However, if the size of sheets is variable, the guiding member has to move in the sheet-width direction to near the corner of the current sheets. That is, it is necessary to provide a moving space extending in the sheet-width direction. Moreover, it is necessary to provide a driving unit that moves the guiding member. This brings an increase of costs and an increase of necessary space for the driving unit. Because a typical driving unit includes a motor and a driving-force transmission mechanism, it is expected to bring a large increase in the number of parts and a large increase in the necessary space.